1. Technical Field
Embodiments described herein relate to system in packages (SiPs) and methods for making SiPs. More particularly, embodiments described herein relate to systems and methods for shielding SiPs from electromagnetic interference.
2. Description of Related Art
An SiP (system in package or system-in-a-package) includes one or more integrated circuits enclosed in a single module (e.g., a single package). The SiP may perform many (or all) of the functions of an electronic system. SiPs are typically used inside smaller electronic devices such as, but not limited to, mobile phones, digital music players, and tablets. An example of an SiP may include several chips (e.g., a specialized processor, DRAM, and/or flash memory) combined with passive components (e.g., resistors and capacitors) mounted on a single substrate. Mounting all the components on the single substrate provides a complete functional unit that can be built in a multi-chip package and few external components may be needed to make the device work. A drawback to SiPs is that any defective chip in the package will result in a non-functional packaged integrated circuit, even if all the remaining modules in the same package are functional.
EMI (“electromagnetic interference”) is the unwanted effects in the electrical system due to electromagnetic (e.g., radio frequency (RF)) radiation and electromagnetic conduction. Electromagnetic radiation and electromagnetic conduction are different in the way an EM field propagates. Conducted EMI is caused by the physical contact of the conductors as opposed to radiated EMI which is caused by induction. Electromagnetic disturbances in the EM field of a conductor will no longer be confined to the surface of the conductor and may radiate away from it. Mutual inductance between two radiated electromagnetic fields may result in EMI.
Due to EMI, the electromagnetic field around the conductor is no longer evenly distributed (e.g., resulting in skin effects, proximity effects, hysteresis losses, transients, voltage drops, electromagnetic disturbances, EMP/HEMP, eddy current losses, harmonic distortion, and reduction in the permeability of the material).
EMI can be conductive and/or radiative and its behavior is dependent on the frequency of operation and cannot be controlled at higher frequencies. For lower frequencies, EMI is caused by conduction (e.g., resulting in skin effects) and, for higher frequencies, by radiation (e.g., resulting in proximity effects).
A high frequency electromagnetic signal makes every conductor an antenna, in the sense that they can generate and absorb electromagnetic fields. In the case of a printed circuit board (“PCB”), consisting of capacitors and semiconductor devices soldered to the board, the capacitors and soldering function like antennas, generating and absorbing electromagnetic fields. The chips on these boards are so close to each other that the chances of conducted and radiated EMI are significant. Boards are designed in such a way that the case of the board is connected to the ground and the radiated EMI is typically diverted to ground. Technological advancements have drastically reduced the size of chipboards and electronics and locating SiPs along with other components closer and closer together. The decreasing distances between components, however, means that chips (e.g., SiPs) are also becoming more sensitive to EMI. Typically electromagnetic shielding is used to inhibit EMI effects. However, EMI shielding for SiPs may be difficult and process intensive to integrate into the SiP structure.
FIG. 1 depicts a side-view cross-sectional representation of an example for providing EMI shielding for an SiP. SiP 100 includes silicon die 102 and passive devices 104 coupled to the upper surface of substrate 106. Substrate 106 may be a two layer substrate (e.g., a substrate with a core and two metal layers). Silicon die 102 and passive devices 104 are encapsulated in encapsulant 108. Terminals 110 may be coupled to the lower surface of substrate 106. Underfill material 112 (e.g., solder resist) may be formed on the lower surface of substrate 106 around terminals 110.
Terminals 110 may couple SiP 100 to printed circuit board (PCB) 114. PCB 114 may be, for example, a multilayer PCB. Shield 116 is formed over encapsulant 108 of SiP 100. Shield 116 is a metal shield. As shown in FIG. 1, to form an EMI shield for SiP 100, shield 116 contacts ground ring 118 at the lower edges of the shield (inside the dotted circles) on the ends (sides) of substrate 106. Ground ring 118 couples shield 116 to outermost terminals 110′ on the lower surface of substrate 106. Terminals 110′ are coupled to routing in PCB 114 that connects the terminals (and shield 116) to ground layer 120 at the bottom-most surface of the PCB. When shield 116 and ground layer 120 are electrically coupled, as shown in FIG. 1, they together form EMI shield 122 (e.g., a Faraday cage) around SiP 100.
A problem that occurs with making the shield structure shown in FIG. 1 is that it is difficult to ensure electrical connection between shield 116 and ground ring 118. FIG. 2 depicts an enlarged cross-sectional representation of an end portion of substrate 106 with shield 116 and ground ring 118 not connected. Typically, SiP 100 is placed on an adhesive surface (e.g., adhesive tape) or in a fixture pocket with raised walls during sputtering (or electroplating) of material for shield 116 to inhibit metal deposition on the lower surface of substrate 106. The adhesive surface or the walls of the fixture pocket may form region 124 around the end portion of substrate 106, as shown in FIG. 2. For example, the adhesive surface may extend up along the side surface of substrate 106 or the walls of the fixture pocket may contact or be very close to the side surface of the substrate.
Region 124 may be inaccessible for metal deposition of shield material on the side surface of substrate 106. The lack of metal deposition may form gap 126 between shield 116 and ground ring 118. In some cases, gap 126 may include a region with a lower thickness of metal deposition (and thus higher electrical resistivity) as compared to other regions of the module. Gap 126 inhibits electrical contact (e.g., metal to metal contact) between shield 116 and ground ring 118. The inaccessibility for metal deposition due to region 124 is a particular problem as ground ring 118 has a small thickness (about 10-15 μm), which provides a small target area for shield 116 to contact. As substrates get thinner and thinner, contacting the ground ring will become even more difficult. Without contact between shield 116 and ground ring 118, as shown in FIG. 2, it is difficult for a complete EMI shield to be formed as there is no electrical contact between the shield and ground layer 120 (shown in FIG. 1). Thus, as shown in FIG. 2, EMI shield 122 is an incomplete shield.